U.S. patent number 10,799,365 [Application Number 16/678,552] was granted by the patent office on 2020-10-13 for bone joint implants.
This patent grant is currently assigned to Loci Orthopaedics Limited. The grantee listed for this patent is Loci Orthopaedics Limited. Invention is credited to Brendan Boland, Gerry Clarke, Amy L. Ladd, Filip Stockmans, Arnold-Peter C. Weiss.
United States Patent |
10,799,365 |
Stockmans , et al. |
October 13, 2020 |
Bone joint implants
Abstract
Bone joint implants are described herein. The bone joint
implants may comprise a metallic proximal platform configured for
translational motion on the trapezium bone; a distal stem
configured for intramedullary engagement with an end of the first
metacarpal bone; an articulating coupling between the proximal
platform and distal stem; and a proximal non-metallic wear surface
and a distal non-metallic wear surface.
Inventors: |
Stockmans; Filip (Heule
Kortrijk, BE), Clarke; Gerry (County Galway,
IE), Weiss; Arnold-Peter C. (Barrington, RI),
Ladd; Amy L. (Stanford, CA), Boland; Brendan (County
Kildare, IE) |
Applicant: |
Name |
City |
State |
Country |
Type |
Loci Orthopaedics Limited |
Upper Newcastle, Galway |
N/A |
IE |
|
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Assignee: |
Loci Orthopaedics Limited
(Upper Newcastle, Galway, IE)
|
Family
ID: |
1000005110336 |
Appl.
No.: |
16/678,552 |
Filed: |
November 8, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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62823392 |
Mar 25, 2019 |
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62823367 |
Mar 25, 2019 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61F
2/4241 (20130101); A61F 2/4261 (20130101); A61F
2002/4258 (20130101); A61F 2002/3006 (20130101); A61F
2002/30663 (20130101); A61F 2002/30935 (20130101); A61F
2002/30685 (20130101) |
Current International
Class: |
A61F
2/42 (20060101); A61F 2/30 (20060101) |
References Cited
[Referenced By]
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WO 2014/077750 |
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May 2014 |
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WO |
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WO 2015/088403 |
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Jun 2015 |
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WO |
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WO 2017/137607 |
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Aug 2017 |
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WO |
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Other References
European Search Report for European Application No. 19167519.8,
dated Sep. 12, 2019 (7 pages). cited by applicant .
International Search Report and Written Opinion for International
Application No. 2019/0058, completed Jun. 25, 2019 (8 pages). cited
by applicant .
International Search Report and Written Opinion for International
Application No. PCT/EP2017/053079, dated Aug. 23, 2017 (11 pages).
cited by applicant .
Crisco, J. et al., "In Vivo Kinematics of the Trapeziometacarpal
Joint During Thumb Extension-Flexion and Abduction-Adduction," J
Hand Surg Am., 2015, vol. 40 (2) 289-96. cited by applicant .
De Aragon, J.S.M. et al., "Early Outcomes of Pyrolytic Carbon
Hemiarthroplasty for the Treatment of Trapezial-Metacarpal
Arthritis," J Hand Surg Am., 2009, vol. 34A (2) 205-12. cited by
applicant .
Krukhaug, Y. et al., "The results of 479 thumb carpometacarpal
joint replacements reported in the Norwegian Arthroplasty
Register," J Hand Surg Am., 2014, vol. 39 (8) 1-7,
http://jhs.sagepub.com/content/early/2014/04/29/1753193413513988.
cited by applicant .
Naidu, S.H. et al., "Titanium Based Joint Arthroplasty: A Finite
Element Analysis and Clinical Study," J Hand Surg Am., 2006, vol.
31 (5) 760-65. cited by applicant .
Pritchett, J.W. et al., "A Promising Thumb Basal Joint
Hemiarthroplasty for Treatment of Trapeziometacarpal
Osteoarthritis," Clinical Orthopaedics and Related Research, 2012,
vol. 470 (10) 2756-63. cited by applicant .
Turker, T. et al., "Trapezio-metacarpal arthritis: The price of an
opposable thumb!," Indian Journal of Plastic Surgery, 2011, vol. 44
(22) 308-16. cited by applicant .
"TIE-IN.TM. Trapezium Implant--A Comprehensive Solution for CMC,"
Wright Medical Group N.V., Jul. 30, 2016. cited by applicant .
Summary of Safety and Effectiveness, TIE-IN.TM. Trapezium, Wright
Medical Group, Nov. 7, 2003. cited by applicant .
International Search Report and Written Opinion for International
Application No. PCT/EP2020/055344, dated May 13, 2020 (12 pages).
cited by applicant .
International Search Report and Written Opinion for International
Application No. PCT/EP2020/055353, dated May 13, 2020 (6 pages).
cited by applicant.
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Primary Examiner: Snow; Bruce E
Assistant Examiner: Hoban; Melissa A
Attorney, Agent or Firm: Bookoff McAndrews, PLLC
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of priority of U.S. Provisional
Application Nos. 62/823,367 and 62/823,392, both filed on Mar. 25,
2019, all of which are incorporated herein by reference in their
entireties
Claims
The invention claimed is:
1. A bone joint implant for a mammalian first carpometacarpal
joint, comprising: a proximal part configured for translational
motion on the trapezium bone, the proximal part including a
platform; a distal part configured for intramedullary engagement
with an end of the first metacarpal bone, the distal part including
a stem and an insert extending into a proximal end of the stem, the
insert including a flange extending proximal of the proximal end of
the stem; and an articulating coupling between the proximal and
distal parts, and the flange including a proximal end surface
limiting movement between the proximal part and the distal
part.
2. The bone joint implant as claimed in claim 1, wherein the insert
is non-metallic and the platform is metallic.
3. The bone joint implant as claimed in claim 2, wherein the
proximal end surface of the flange has a concave curvature.
4. The bone joint implant as claimed in claim 3, wherein the
platform includes a distal end surface having a convex
curvature.
5. The bone joint implant as claimed in claim 4, wherein the flange
is annular.
6. The bone joint implant as claimed in claim 1, wherein the
articulating coupling is a ball and socket coupling, and the
proximal part includes the ball, and the insert includes the
socket.
7. The bone joint implant as claimed in claim 6, wherein the ball
extends distally of the flange.
8. A bone joint implant for a mammalian first carpometacarpal
joint, comprising: a proximal part configured for translational
motion on the trapezium bone, the proximal part including a
metallic platform having a proximal end surface having a concave
curvature, and a distal end surface having a convex curvature; a
distal part configured for intramedullary engagement with an end of
the first metacarpal bone, the distal part including a metallic
stem and a non-metallic insert extending into a proximal end of the
stem, the insert including a flange extending proximal of the
proximal end of the stem; and a ball and socket coupling between
the proximal and distal parts, the proximal part including the
ball, and the insert including the socket, and the flange including
a proximal end surface limiting movement between the proximal part
and the distal part.
9. The bone joint implant as claimed in claim 8, wherein the
proximal end surface of the flange includes a concave surface.
10. The bone joint implant as claimed in claim 8, wherein the
proximal end surface of the flange is annular.
Description
INTRODUCTION
The invention relates to an implant for a bone joint. In some
examples it relates to implants in which there are multiple axes of
rotation, such as those in which there is a dual axis
hemiarthroplasty with two axes of rotation such as in the hand or
elbow. However, in other aspects the invention relates to a
uni-axial implant such as a hip joint implant.
An example of an implant with multiple axes of rotation is one for
a first carpometacarpal joint for spacing a trapezium bone from a
first metacarpal bone. In this case there is translational motion
of a saddle-shaped surface of a proximal implant part over the
trapezium and three-dimensional rotational movement of the distal
part due to an articulated coupling such as a ball-and-socket
joint. An example of such an implant is described in WO2017/137607
(NUIG).
In such an implant the point of motion may be at, or between, two
points, concurrently or independently. Depending on the
biomechanics of the joints into which the implant has been inserted
the main force of motion may change rapidly and abruptly between
the two points. FIGS. 1(a) and (b) illustrate a dual axis implant
providing two axes of rotation: Ab-Ad (Axis 1) in the base of the
metacarpal and Ex-Fl (Axis 2) over the surface of the trapezium to
recreate the native axes of rotation of the joint. A radiolucent
stem is used in FIG. 1(a) to demonstrate the location of the ball
and socket in the metacarpal.
Impingement in an orthopaedic implant may be caused any time there
is a decrease in the space between the two elements causing one
element to impact on the other, for example, the head of an
articulating hemiarthroplasty impinging on the stem.
Uncontrolled impingement is a cause of poor outcomes of implants.
Referring also to uni-axial implants such as prosthetic hip
arthroplasty; it can lead to instability, accelerated wear, and
unexplained pain. Impingement is influenced by prosthetic design,
component position, biomechanical factors, and patient variables.
Uncontrolled impingement is linked to dislocation and accelerated
wear comes from implant retrieval studies. Operative principles
that maximize an impingement-free range of motion include correct
combined acetabular and femoral anteversion and an optimal
head-neck ratio. Operative techniques for preventing impingement
include medialization of the cup to avoid component impingement and
restoration of hip offset and length to avoid osseous
impingement.
To illustrate these problems FIG. 1 (c) is an image from Brown T D,
Callaghan J J. Impingement in Total Hip Replacement: Mechanisms and
Consequences. Curr Orthop. 2008; 22(6):376-391. These images show
Finite Element Analysis of a constrained-liner THA, showing stress
contours at the instant of incipient dislocation (A, B). FIG. 1(d),
also from Brown T D, Callaghan J J. Impingement in Total Hip
Replacement: Mechanisms and Consequences. Curr Orthop. 2008;
22(6):376-391, shows a 3D Finite Element model of total hip
dislocation, illustrating the initial relative position of the
implant components (left), the acetabular component bearing surface
stress during stable articulation (centre), and the corresponding
stresses just prior to a posterior dislocation event (right).
The invention is directed towards providing an improved implant
with controlled impingement, or at least reduced effects if
impingement occurs.
SUMMARY OF THE INVENTION
The present disclosure includes bone joint implants. For example,
the present disclosure includes a bone joint implant for a
mammalian first carpometacarpal joint comprising a metallic
proximal platform configured for translational motion on the
trapezium bone; a distal stem configured for intramedullary
engagement with an end of the first metacarpal bone; an
articulating coupling between the proximal platform and distal
stem; and a proximal non-metallic wear surface and a distal
non-metallic wear surface.
According to some examples herein, the proximal non-metallic wear
surface may form a buffer surface that prohibits contact between
the proximal platform and the stem during articulation; may include
a concave curvature; and/or may form an annular surface. The
proximal platform may include a distal end surface having a convex
curvature. In at least one example, the distal non-metallic wear
surface may be spherically shaped.
In some examples, the implant may include a unitary non-metallic
wear member, and the proximal non-metallic wear surface and the
distal non-metallic wear surface may be formed on the unitary wear
member. The unitary non-metallic wear member may be an insert
received in a proximal end surface of the stem, and the insert may
include a proximal portion extending proximally of the proximal end
surface of the stem. In some examples, the proximal portion may be
a flange of the insert and the flange may include the proximal
non-metallic wear surface, and/or the articulating coupling may be
a ball and socket coupling, and the insert may form the socket of
the ball and socket coupling.
The present disclosure also includes a bone joint implant for a
mammalian first carpometacarpal joint, comprising a proximal part
configured for translational motion on the trapezium bone, the
proximal part including a platform; a distal part configured for
intramedullary engagement with an end of the first metacarpal bone,
the distal part including a stem and a wear surface located
proximal the stem; and an articulating coupling between the
proximal and distal parts, the wear surface being further located
to limit articulation and prohibit contact between the platform and
the stem. In at least one example, the wear surface may be
non-metallic and the platform may be metallic. The wear surface may
include a concave curvature and/or may form an annular surface. The
platform may include a distal end surface having a convex
curvature.
In some examples, the implant may include an insert received in a
proximal end surface of the stem, wherein the wear surface is
formed on the insert. The insert may include a proximal flange,
wherein the wear surface is formed on the flange. Additionally, the
articulating coupling may be a ball and socket coupling, and the
ball may form a part of the proximal part, and the socket may be
formed by the insert. In at least one example, the ball may extend
distally of the flange.
The present disclosure also includes a bone joint implant for a
mammalian first carpometacarpal joint, comprising a proximal part
configured for translational motion on the trapezium bone, the
proximal part including a platform; a distal part configured for
intramedullary engagement with an end of the first metacarpal bone,
the distal part including a stem and an insert extending into a
proximal end of the stem, the insert including a flange extending
proximal of the proximal end of the stem; and an articulating
coupling between the proximal and distal parts, and the flange
including a proximal end surface limiting movement between the
proximal part and the distal part. In at least one example, the
insert may be non-metallic and the platform may be metallic. The
proximal end surface of the flange may have a concave curvature
and/or the platform may include a distal end surface having a
convex curvature. In at least one example, the flange may be
annular. Additionally, the articulating coupling may be a ball and
socket coupling, the proximal part may include the ball and the
insert may include the socket, and/or the ball may extend distally
of the flange.
The present disclosure also includes a bone joint implant for a
mammalian first carpometacarpal joint, comprising a proximal part
configured for translational motion on the trapezium bone, the
proximal part including a metallic platform having a proximal end
surface having a concave curvature, and a distal end surface having
a convex curvature; a distal part configured for intramedullary
engagement with an end of the first metacarpal bone, the distal
part including a metallic stem and a non-metallic insert extending
into a proximal end of the stem, the insert including a flange
extending proximal of the proximal end of the stem; and a ball and
socket coupling between the proximal and distal parts, the proximal
part including the ball, and the insert including the socket, and
the flange including a proximal end surface limiting movement
between the proximal part and the distal part. According to some
examples, the proximal end surface may include a concave surface
and/or the proximal end surface may be annular.
DETAILED DESCRIPTION OF THE INVENTION
The invention will be more clearly understood from the following
description of some embodiments thereof, given by way of example
only with reference to the accompanying drawings in which:
FIGS. 1(a) and 1(b) are diagrams showing in the prior art different
axes of motion for an implant for a first carpometacarpal joint for
spacing a trapezium bone from a first metacarpal bone, as discussed
above in the Introduction;
FIG. 1(c) is a Finite Element Analysis (FEA) diagram showing prior
art impingement in a hip joint, and FIG. 1(d) is an FEA diagram
also of a hip joint showing impingement and egress sites in the
prior art, also as discussed above in the Introduction;
FIG. 2 is an image showing in perspective an implant of the
invention, both proximal and distal parts;
FIG. 3(a) is a cross-sectional view through the implant distal
part, and FIG. 3(b) is a perspective view of the stem of the distal
part, FIG. 3(c) is a perspective view of the complete distal part,
and FIG. 3(d) is a perspective view of the proximal part;
FIG. 4 is a set of views showing a portion of the distal part of
the implant, including top plan, end, perspective, and
cross-sectional views;
FIG. 5 is a set of three pairs of end and sectional views showing
the implant at different relative positions between its proximal
and distal parts;
FIG. 6 is a diagram illustrating the allowed cone of motion between
the proximal and distal parts;
FIGS. 7(a) and 7(b) illustrate cross-sectional views of an implant,
showing the areas of material stress;
FIGS. 8(a) and 8(b) are sectional views showing an implant with an
interface coating on the proximal part; and
FIG. 9 is a cross-sectional view of an implant for a hip joint,
with a flange to increase surface area to reduce contact
stresses.
TERMS
"Intramedullary engagement" means engagement within a medullary
cavity formed or existing in the bone, where the cavity is
generally but not exclusively formed along a longitudinal axis of
the bone. In one embodiment, the intramedullary engagement fixture
comprises a screw or nail or interference-fit stem, although other
intramedullary fixtures are known. Typically, the screw is
externally threaded. Intramedullary fixtures are sold by Smith
& Nephew, Zimmer, Synthes and other suppliers. The engagement
anchors the implant to the bone. In one embodiment, the medullary
cavity is formed in a position that is offset towards a volar
direction. The medullary cavity may be formed in a position offset
from the anatomical and or biomechanical axis of the bone.
"Non-engaging abutment" means that the proximal part is not fixed
to the first bone, but is configured to abut the end of the bone in
a manner that allows translational movement thereof. How this is
achieved depends on the joint being treated and the specific
anatomy of the first bone. As an example, when the joint is a
carpometacarpal joint in the thumb, the end of the trapezium bone
has a twisted saddle shape (see FIG. 2 of Turker et al, Indian J
Plast Surg. 2011, 44(2): 308-316) and the platform is configured to
rest upon this saddle and allow translational movement of the
platform across the saddle. Thus, in this embodiment, the curved
saddle-shaped platform typically has a concave-convex shape, which
has a concave curvature along a longitudinal aspect, and a convex
curvature along a lateral aspect. The curved saddle-shaped platform
may have a concave and convex curvature in both the longitudinal
and lateral aspects, i.e., both the length and width directions (as
shown in the figures, e.g., FIGS. 2 and 4). This shape has been
shown to provide an engagement that closely mimics the
physiological situation and allows for natural flexion-extension
articulation. When discussing curvature in this disclosure (e.g.
concave or convex), the point of reference is from outside the
structure (implant) or component, not from inside the structure or
component.
"Translational movement of the second bone in relation to the first
bone" means non-pivoting movement of the second bone in relation to
the first bone. This can also be described as sliding movement. An
example is the involuntary translational movement of the metacarpal
in relation to the trapezium in the thumb carpometacarpal joint,
which contributes significantly to extension-flexion articulation
of the thumb. The implant of the invention facilitates such
translational movement by employing a proximal part that is
configured to non-engagingly abut the first bone.
"Articulating coupling" means a coupling that allows articulation
between the first and second parts of the implant. The specific
type of coupling employed in the implant depends on the joint that
is being treated with the implant, and in some cases the indication
or severity of the indication. For example, when the implant is for
treatment of an arthritic hinge joint, for example an elbow joint,
the implant will generally comprise a hinge joint coupling. When
the implant is for treatment of a saddle joint, for example a
carpometacarpal joint, the implant will generally comprise a ball
and socket joint or a universal joint. "Controlled articulation"
means that the articulation is constrained to specific types of
articulation.
"Abutting platform" means a base that abuts the end of the first
bone (for example the end of the trapezium) so that translational
(i.e. sliding) movement of the platform in relation to the end of
the bone is allowed. The bone is not fixed to the platform. The
platform may be configured to conform to a surface of the top of
the bone. In one embodiment, the platform is shaped to mimic an end
of the second bone, so as to allow the same range of movements as
the natural healthy joint, including translational movement. In the
case of the carpometacarpal joint, where the end of the first bone
(trapezium) has a twisted saddle topography, the platform may be
shaped to conform to the twisted saddle to allow one or more or all
of the following range of movements of the first metacarpal in
relation to the trapezium, flexion, extension, abduction,
adduction, internal rotation, external rotation, opposition,
circumduction, and translation.
DESCRIPTION OF THE EMBODIMENTS
Referring to FIGS. 2 to 7 an implant 1 has a distal part with an
insert 100 in a stem 110, and a proximal part 120.
In this case the implant 1 is for a mammalian first carpometacarpal
joint as shown in FIG. 1(a) for spacing a trapezium bone of the
joint from a first metacarpal bone of the joint while allowing
translational movement of the first metacarpal bone in relation to
the trapezium bone. The distal part 110 is configured for
intramedullary engagement with an end of the first metacarpal bone.
The proximal part 120 has a curved saddle-shaped platform 122 with
a proximal-facing surface 124 for sliding on or traversing the
trapezium bone. An articulating coupling (e.g., ball-and-socket)
comprises a neck 123 bridging the saddle 122 to a ball 121, as is
known. This allows controlled articulation of the trapezium and
first metacarpal bones.
The insert 100 has a buffer interface feature (i.e., buffer
surface), in this case a flange 105 with a contoured
proximally-facing surface 101, which may be annular as shown in
FIG. 5. Distally of this surface there is a shoulder 102 which acts
as a key for engaging the insert 100 in the stem 111 (see FIG. 5)
and preventing rotation of the insert in the stem, and surrounding
a socket 103 with a rim 106 to receive the articulated coupler ball
121. There is snap-fit engagement of the ball 121 (see especially
FIG. 3(d) and FIG. 5) in the socket 103, behind the socket's rim
106, to enable the assembly of an articulating hemiarthroplasty
intra-operatively, and it may also prevent disassembly of the
device in vivo. The socket can be central or offset in any
direction or angle as needed.
Further distally, the insert 100 comprises an annular locking rim
104 for snap-fitting into a corresponding groove 116 of the stem
111 recess 115 which accommodates the insert 100. Engagement of the
insert 100 into the stem 111 is effective due to the resilience of
the insert material and the fact that there is comprehensive
surface-to-surface contact in a snap-fitting manner between the rim
104 and its corresponding engagement surface within the stem 111.
This snap-fit engagement of the insert 100 and stem 111 enables the
assembly of an articulating hemiarthroplasty intra-operatively, and
it my also prevent disassembly of the device in vivo. The insert is
keyed by the shoulder 102 to prevent rotation and potential
consequent back side wear.
The flange 105 (and in this case the whole insert 100) is of a
resilient polymer material which is preferably a polymer, such as
UHMWPE (in any of its forms, possibly including vitamin E) or PEEK,
in any of its forms. In such case, the insert 100 may be referred
to as a unitary non-metallic wear member. It may alternatively be
of other materials commonly used in orthopaedics such as Pyrocarbon
(PyC), or ceramic depending on the wear patterns expected of the
construct. The insert 100 is of a material which is different from
the metal material of the unitary proximal part 120 (saddle 122,
neck 123, and articulated coupler ball 121), hence avoiding any
Galvanic-type interactions which may cause excessive wear and/or
chemical reactions which give rise to contaminants. Likewise, the
(polymer) material of the insert is different from the metal
material of the stem 111 for the same reasons. In general
metal-to-metal contact interfaces are avoided in the implant. While
a polymer material is good for wear, the biomechanical advantages
of the flange i.e., breaking up the two axes of rotation, may be
more important, and as such the flange could possibly be made of
any suitable material. An example would be where the insert (or
"liner") is made of a ceramic material, but the head is made of
PEEK, which would still enable a snap fit engagement for the
articulating coupling. It is generally preferred that the flange
and the socket are not of a relatively hard material as that might
not permit a snap fit for anything other than a material with low
modulus/high resilience. This may be the other way around, for
example, if the head is a polymer and the liner is a ceramic, the
soft polymer material may still snap fit into the hard ceramic
socket.
Thus, as discussed above, the implant 1 may include at least one
non-metallic wear surface. The non-metallic wear surface may be
present on any portion of the implant where a surface of the stem
110 and a surface of the platform 122 may engage, as shown in FIG.
5. In the case when the stem 110 includes an insert 100, the insert
100 may include at least one non-metallic wear surface. The at
least one non-metallic wear surface may include a shaped surface,
for example a surface with a convex curvature, and/or may form an
annular surface. The platform may include a distal end surface
having a corresponding shape, for example concave. Alternatively,
if the at least one non-metallic wear surface has a concave
curvature, the platform may include a distal end surface having a
convex curvature.
The flange material resilience is preferably sufficient to allow
compression in use, to an extent desired to achieve gradual
conversion of motion between the axes. For this implant, for the
thumb, the thickness of the flange 105 is preferably in the range
of 0.5 mm to 4 mm, and preferably 1.0 mm to 3.0 mm. The implant may
be provided as a kit in which there is the proximal part 122, the
stem 111 of the distal part 110, and a range of two or more inserts
each of which fits into the stem 111 but has a different flange
thickness. The flange thickness sets the range of relative motion
allowed, and in the example illustrated in FIG. 6 it is 40.degree..
In general, the flange is preferably configured to provide a cone
of motion in the range of 30.degree. and 60.degree. of the distal
part about the proximal part. This allows the surgeon to choose the
desired cone of motion. The implant thus achieves a predictable
wear pattern. Also, decreasing the cone of motion reduces the
chances of dislocation. It should be noted that for this type of
joint, multi-axial, the full range of motion is actually about
80.degree. when one takes into account the sliding motion of the
proximal part over the trapezium bone. The illustration of FIG. 6
is based on the proximal part being static, for illustration
purposes.
Moreover, the flange 105 contoured proximally-facing surface 101 is
configured to match a corresponding mating distal surface 125 of
the saddle 122, to cause the motion of forces between the two axes
of motion to be limited in a step-wise manner, i.e., limiting
movement between the parts 110 and 120. Hence, there is not an
abrupt change in force, or "flip-flop" between the two axes. The
mating surfaces 101 and 125 provide a large surface area for
contact between the parts 110 and 120 as illustrated in FIGS. 5 and
7.
By having a load bearing surface 101 interposed between the axes,
the forces are distributed in a more controlled, more natural, and
more physiological manner. The relative motion around the
articulated coupling is limited in one example to about 40.degree.,
as illustrated in FIG. 6. This extent of motion is sufficient for
use of the implant after deployment, but it also helps to ensure
that there are not excessive impact forces between the surfaces and
there is a smooth transition between the axes shown in FIGS. 1(a)
and 1(b).
The liner snap-fit element 104 enables easy and effective assembly
into the stem 111. Also, the liner snap-fit socket 103 facilitates
the capture of a mating ball to form the ball-and-socket joint in a
manner which is advantageous because of the resilience of the
material of the insert 100. As shown in the figures, the socket 103
may be spherically shaped to receive the ball of the
ball-and-socket joint.
The flange 105 surface 101 is contoured to match the geometry of
the head component to maximize surface contact and hence minimize
liner wear.
The insert 100 is replaceable from within the stem, i.e., it can be
removed, and another inserted in its place in the case of excessive
wear. Insert 100 may be installed and/or removed with an
appropriate tool or tool set.
The insert 100 advantageously limits the extent of relative
rotation in the abduction-adduction and flexion-extension planes.
As shown in FIGS. 5 and 6 the saddle 122 has less freedom to rotate
upwardly in this view and when contact is made with the surface
insert 100 there is full-surface contact between the saddle 122 and
the contoured surface 101. Preferably, the flange's contoured
surface is tapered radially and distally and the saddle's
corresponding mating surface is tapered radially and
proximally.
On the lower side as viewed in FIG. 5, there is a smaller meeting
surface area, but the same effects and advantages apply. It will be
appreciated that the insert 100 causes the impingement issue to be
ameliorated.
Referring to FIGS. 7(a) and 7(b), the shaded areas receive maximum
material stress between the ball and socket interface. In FIG.
7(b), the saddle 122 of proximal part 120 is not in contact with
the flange 105 of insert 100. As a result, all of the material
stresses are concentrated at the ball and socket interface (shown
in FIG. 7(b) as the stress region on ball 121). FIG. 7(a)
illustrates a portion of proximal part 120, e.g., the
distally-facing side of the saddle 125, in contact with the flange
105 of insert 100 and proximally facing surface 101. This increased
contact area, shown as an additional stress region in FIG. 7(a)
allows for a wider distribution of material stresses. The insert
100 may be thickest at the portion with such increased contact
area. This wider and more even distribution of stress load on the
implant 1, reduces stress concentration at the ball and socket
interface, shown at ball 121, and may prolong the life of the
implant 1.
Alternative Examples
It is also envisaged that the implant may have a buffer interface
(i.e., buffer surface) which includes a feature in addition to or
instead of a flange, and/or which is not necessarily on an insert
in the distal part. For example, the proximal part may have a
buffer interface on the distal-facing surface, which interface
engages the distal part with a large surface area. Such an
interface may be a coating of a thickness in the range of 0.5 mm to
3.0 mm, and preferably 1.0 mm to 2.0 mm for example. The interface
is preferably of a resilient material, such as any of the polymers
mentioned in the description above. In this case it is envisaged
that the distal part may not have a flange in some examples, in
which case the proximal-part interface feature engages the distal
part stem directly.
Referring to FIGS. 8(a) and 8(b) an implant 150 is also for a thumb
and comprises a distal part with a stem 151 and an insert 152 to
receive a ball 153 of a proximal part 154. The distally-facing side
of the proximal part 154 has a buffer interface namely a coating
157. This is provided to achieve at least some of the benefits of
the flange of the previous embodiments. The coating 157 provides a
buffer effect which reduces the cone of motion and provides an
increased surface area for distribution of forces across the
construct.
It is envisaged that the implant distal part may include the
physical features of the insert in an integral manner. Or, the
flange 105 may be provided as a discrete item. Also, an insert
could alternatively be threaded for engagement in the stem rather
than being snap-fitted.
A stem with an integral flange may comprise a hard material and a
coupling ball may be made of a softer material. The flange is in
relation to the base of the stem and metacarpal bone.
The flange may be an integral part of the stem. It would preferably
have the advantageous features of having a surface contoured to
provide a large surface area for contact with the contacting
part.
It is envisaged that there may alternatively be a resilient ball
and a high modulus socket.
In examples above, the contour of the flange matches the
distally-facing surface 125 of the saddle 122, however, the
surfaces may be configured otherwise. For example, the
distally-facing surface 125 of the saddle 122 may have a convex
curvature as shown in the figures.
The distraction distance, i.e., the distance between two bones
post-implantation of a device, can be modified by increasing the
height of the saddle head as shown in FIGS. 5 and 6 for example.
The cone of motion will not be affected as the flange will be of
the same geometry. Alternatively, the flange can be thicker or
thinner which will affect the cone of motion. In some clinical
uses, for example, a patient who frequently dislocates an implant,
a surgeon may choose during revision surgery to use a flanged
insert with a very thick flange, to decrease the range of motion,
potentially decreasing the likelihood of dislocation by increasing
the jump distance (the distance a ball must move out of a socket in
order to dislocate).
It is envisaged that the insert may be engaged in the stem without
a mechanical/physical engagement feature, and may have only an
adhesive bond.
Uniaxial Implant Examples
As noted in the Introduction with reference to FIGS. 1 (c) and (d),
hip implants can have issues with impingement of the liner around
the ball. We also describe an implant with a flange which may
improve the outcomes of hip implants. FIG. 9 illustrates an implant
200 with a neck 210 with a ball, and a socket 202 having a flange
203 extending radially around the socket's entrance. The material
of the flange 203 is of a resilient material such as UHMWPE, PEEK,
ceramic or another material commonly used in orthopaedic implants.
The flange 203 is part of a liner for the socket 202, and the liner
is attached to the remainder of the socket (distal) part by snap
fit, taper, or some other suitable locking mechanism. The flanged
liner 203 has a geometry with an interfacing surface matching that
of the impinging component, in this case the neck 201 below the
ball. FIG. 9 shows two positions for the neck 201, showing that
there is an angular freedom of movement of 110.degree.. This
arrangement ameliorates the impingement forces over the socket 202,
dispersing stress forces across the interface surface 204 and
helping to prevent dislocation. Hence the flange 203 not only
provides a much larger interfacing/contacting surface 204, but it
is also of a more resilient material so that contact forces are
absorbed.
As noted above, while a polymer liner and metal ball is one
preferred arrangement, there may alternatively be a metal liner and
poly ball, a ceramic liner and poly ball, a poly liner and ceramic
ball in any suitable combination depending on whether the wear
pattern or biomechanical axis management is more important.
The material of the flange 203 is resilient, preferably a polymer
such as UHMWPE, PEEK or ceramic. The insert is attached to the
remainder of the socket part by press fit, threads, or snap fit.
The distal part and proximal parts in total implants of the joints
may be reversed depending on the joint and the implant.
The flange may be contoured to match the geometry of any part of a
mating component. For example, in shoulder implants, the flange may
be concave to mate optimally with the relative geometry of the neck
of an implant. Similarly, in a hip implant, the flange may be
convex to match the geometry of a neck component.
The invention is not limited to the embodiments described but may
be varied in construction and detail.
* * * * *
References